Solid golf ball

ABSTRACT

The present invention provides a golf ball, particularly a solid golf ball having good shot feel and excellent controllability at approach shot, while keeping the characteristics inherent to the solid golf ball, i.e. excellent flight performance and excellent durability. The present invention relates to a solid golf ball comprising a core and a cover formed on the core, wherein the cover has a two-layer structure consisting of an inner cover and an outer cover formed on the inner cover, and the core is formed from a rubber composition comprising 0.05 to 5 parts by weight of an organic sulfide compound, based on 100 parts by weight of a base rubber.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP96/02354 which has an Internationalfiling date of Aug. 23, 1996 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a solid golf ball. More particularly,it relates to a solid golf ball having good shot feel and excellentcontrollability at approach shot, while keeping the characteristicsinherent to the solid golf ball, i.e. excellent flight performance andexcellent durability.

BACKGROUND OF THE INVENTION

Hitherto, there have been mainly produced two types of golf balls. Theone is a solid golf ball, such as a two-piece golf ball, which iscomposed of a core formed from integrally molded rubber material and athermoplastic resin cover (e.g. ionomer resin cover) formed on the core.The other is a thread wound golf ball which is composed of a solid orliquid center, a thread rubber layer formed by winding thread rubberaround the center and a cover of ionomer resin or balata etc. having athickness of 1 to 2 mm, covering on the thread rubber wound layer. Thesolid golf ball, when compared with the thread wound golf ball, hasbetter durability and better flight performance because of largerinitial velocity when hitting and longer flight distance. On the otherhand, the solid golf ball exhibits hard shot feel when hitting, andshows difficulty to put spin on the ball, thus poor controllability atapproach shot. The difficulty of putting spin on the ball comes from thestructural features of the solid golf ball that an initial velocity isvery high, when hitting, and a contact area of the ball with a hittingface of a golf club is very small.

In order to solve the problem, the cover is made from softer material tomake the outer portion of the golf ball soft. However, the reboundcharacteristics of the golf ball are less than those of conventionalcover and deteriorate flight distance inherent to the two-piece solidgolf ball.

In order to solve the above problem, it is proposed that the cover ismade two-layered, of which each layer is formed from a differentmaterial. For example, Japanese Patent Kokai Publication No. 244174/1992proposes that a thermoplastic material comprising a block copolymer ofamides is used in the inner cover layer and a thermoplastic resin isused in the outer cover layer. However, the resulting golf ball has poorrebound characteristics and reduces flight distance, because the blockcopolymer of amides has low rigidity and is used in the inner portion ofthe cover. Japanese Patent Kokai Publication No. 343718/1994 proposesthe use of a hard high-acid content ionomer resin in an inner cover of atwo-layer structured cover golf ball to increase the fight distance. Thehard high acid content ionomer resin, which has high rigidity, is usedin the inner cover, but the resulting golf ball has not hardnesssufficient to increase a golf ball velocity because of softening theouter cover layer. It is required to make the inner cover layer harderto increase the ball velocity.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a schematic cross section illustrating one embodiment of thegolf ball of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to solve the above problem, the present inventors haveintensively studied and have found that rebound characteristics of acore are improved by forming a core from a rubber composition comprising0.05 to 5 parts by weight of an organic sulfide compound, based on 100parts by weight of a base rubber, thereby improving the reboundcharacteristics of the resulting golf ball.

The present invention will be described in detail hereinafter. In thegolf ball of the present invention, a two-layer structured cover layer 2and 3 is formed on a core 1. The core is obtained by vulcanizing orpress-molding a rubber composition. The rubber composition comprises abase rubber, a crosslinking agent, a co-crosslinking agent, an organicsulfide compound and the like.

The base rubber may be natural rubber and/or synthetic rubber which hasbeen conventionally used for solid golf balls. Preferred iscis-1,4-polybutadiene rubber containing a cis-1,4 bond of not less than40%. The polybutadiene rubber may be mixed with natural rubber,polyisoprene rubber, styrene-butadiene rubber, EPDM, and the like.

The crosslinking agents may be an organic peroxide such as dicumylperoxide, t-butyl peroxide and the like. Preferred organic peroxide isdicumyl peroxide. An amount of the organic peroxide is from 0.3 to 5.0parts by weight, preferably from 0.5 to 3.0 parts by weight, based on100 parts by weight of the base rubber.

The co-crosslinking agent may be a material which has beenconventionally used for solid golf balls. It includes a metal salt ofunsaturated fatty acid, particularly mono or divalent metal salts ofunsaturated fatty acids having 3 to 8 carbon atoms (e.g. acrylic acid,methacrylic acid, etc.). Preferred is zinc acrylate because it impartshigh rebound characteristics to the resulting golf ball. An amount ofthe co-crosslinking agent is preferably 10 to 40 parts by weight, basedon 100 parts by weight of the base rubber. When the amount of theco-crosslinking agent is larger than 40 parts by weight, the core is toohard and thus shot feel is poor. On the other hand, when the amount ofthe co-crosslinking agent is smaller than 10 parts by weight, reboundcharacteristics are degraded.

It is required that the rubber composition used for making the solidgolf ball of the present invention comprises an organic sulfidecompound, in addition to the above components. Examples of the organicsulfide compound include polysulfides having 2 to 4 sulfur atoms, suchas diphenyl polysulfide, dibenzyl polysulfide, dibenzoyl polysulfide,dibenzothiazoyl polysulfide, dithiobenzoyl polysulfide and the like, andtetraalkylthiuram sulfide having the formula: ##STR1## wherein n is aninteger of 1 to 4, and R and R' represent CH₃, C₂ H₅, C₄ H₉, C₆ H₅ orethyleneoxy. These organic sulfide compounds may be used alone or incombination of two or more thereof. An amount of the organic sulfidecompound is from 0.05 to 5 parts by weight, preferably from 0.1 to 2parts by weight, based on 100 parts by weight of the base rubber. Whenthe amount of the organic sulfide compound is smaller than 0.05 parts byweight, the technical effect of the organic sulfide compound does notsufficiently exhibit. On the other hand, when the amount of the organicsulfide compound is larger than 5 parts by weight, the technical effectis not further improved.

The rubber composition for the core of the present invention can furthercontain other components which have been conventionally used forpreparing the core of solid golf balls, such as inert filler (such aszinc oxide, barium sulfate, calcium carbonate), antioxidant and thelike.

The resulting core obtained by press-molding the rubber compositionusing the method and condition which have been conventionally used forpreparing the core of solid golf balls preferably has a diameter of 32to 37.5 mm. In order to obtain the desired shot feel, the corepreferably has a deformation amount of 3.5 to 6.0 mm, when applying froman initial load of 10 kg to a final load of 130 kg on the core. When thedeformation amount is smaller than 3.5 mm, the core is too hard. On theother hand, when the deformation amount is larger than 6.0 mm, the coreis too soft.

Then, an inner cover 2 and an outer cover 3 are covered on the core 1.The inner cover material may be ionomer, polyester, nylon and the like.These inner cover materials may be used alone or in combination of twoor more thereof. It is preferable that the inner cover material has aflexural modulus of 3,000 to 7,000 kgf/cm² and a thickness of 1.0 to 3.0mm in view of shot feel and flight performance.

The outer cover material may be materials which have been conventionallyused for forming the cover of solid golf balls, such as ionomer resinand the like. These outer cover materials may be used alone or incombination of two or more thereof. It is preferable that the outercover material has a flexural modulus of 1,000 to 2,800 kgf/cm² and athickness of 1.0 to 3.0 mm in view of shot feel and flight performance.It is preferable that total thickness of the inner cover and the outercover is within the range of 2.0 to 5.0 mm.

The cover used in the present invention (the inner cover 2 and the outercover 3) may optionally contain pigments (such as titanium dioxide,etc.), and the other additives such as a UV absorber, a photostabilizerand a fluorescent agent or a fluorescent brightener, etc., in additionto the cover material, as long as the addition of the additives does notdeteriorate the desired performance of the golf ball cover.

The cover layer of the present invention is formed by a conventionalmethod for forming golf ball cover well known in the art, such asinjection molding, press molding and the like. The method comprises thesteps of forming the inner cover 2 on the core 1 and forming the outercover 3 on the inner cover 2. When forming the outer cover 3, manydepressions called "dimples" are generally formed on the surface of thegolf ball. Both the inner and outer cover layer preferably have athickness of 1.0 to 2.3 mm. In the golf ball of the present invention,paint finishing may be provided on the surface after cover forming forserving commercial sell.

According to the present invention, a core which is soft and has goodrebound characteristics is obtained by using an organic sulfidecompound, thereby providing a golf ball having good shot feel andexcellent flight performance.

EXAMPLES

The following Examples and Comparative Examples further illustrate thepresent invention in detail but are not to be construed to limit thescope thereof.

Formulation

(a) Core composition

Each spherical core having a diameter of 35.6 mm was obtained by mixingthe following core composition and press-molding the mixture.

                  TABLE 1    ______________________________________                            Comparative    Example                 Example    Kind    1      2      3    4    5     1    2    3    ______________________________________    BR-01 *1            100    100    100  100  100   100  100  --    Zinc    26     28     26   26   26    26   24   --    acrylate    Zinc oxide            30     29     30   30   30    30   31   --    Dicumyl 2.0    2.0    2.0  2.0  2.0   2.0  2.0  --    peroxide    Antioxidant            0.5    0.5    0.5  0.5  0.5   0.5  0.5  --    *2    Diphenyl            0.5    2.0    0.5  0.5  0.5   --   --   --    disulfide    ______________________________________     Vulcanizing condition: 150° C. × 25 min.

(b) Inner cover composition

Each inner cover layer having a thickness of 1.7 mm was formed byinjection molding the following cover composition on the core.

                  TABLE 2    ______________________________________    Kind            A     B         C   D    ______________________________________    Hi-milan *3 1605                    --    40        38  --    Hi-milan 1706   --    40        38  50    Hi-milan AM7317 40    --        --  --    Hi-milan AM7318 60    --        --  --    Rilsan ANN OD *4                    --    20        --  --    Hi-milan 1855   --    --        50  --    IOTEC 8000 *5   --    --        --  50    ______________________________________     *1: Polybutadiene (trade name "BR01") from Japan Synthetic Rubber Co.,     Ltd.     *2: Antioxidant (trade name "Yoshinox 425") from Yoshitomi Pharmaceutical     Inds., Ltd.     *3: Himilan (trade name), ionomer resin, manufactured by Mitsui Du Pont     Polychemical Co., Ltd.     *4: Rilsan ANN OD (trade name), polyamide elastomer, manufactured by Tora     Co., Ltd.     *5: IOTEC 8000 (trade name), ionomer resin, manufactured by Exxon Chemica     Co.

                  TABLE 3    ______________________________________    Kind          E     F           G   H    ______________________________________    Hi-milan 1652 50    --          25  30    Hi-milan 1855 50    --          25  30    Hi-milan 1557 --    50          --  --    Hi-milan 1605 --    50          --  --    Hi-milan 1706 --    --          25  20    Hi-milan 1707 --    --          25  20    ______________________________________

                  TABLE 4    ______________________________________    Example No.  1       2       3     4     5    ______________________________________    Inner cover composition                 A       B       A     C     D    Flexural modulus                 4400    5100    4400  3000  4200    (kgf/cm.sup.2)    Outer cover composition                 E       E       G     H     H    Flexural modulus                 1300    1300    2800  2600  2600    (kgf/cm.sup.2)    W#1 Carry (yard)                 221     219     222   219   221    Shot feel    ∘                         ∘                                 ∘                                       ∘                                             ∘    Controllability at                 ∘                         ∘                                 ∘                                       ∘                                             ∘    approach shot    ______________________________________

                  TABLE 5    ______________________________________    Comparative Example No.                    1          2      3(*1)    ______________________________________    Inner cover composition                    A          A      --    Flexural modulus                    4400       4400   --    (kgf/cm.sup.2)    Outer cover composition                    E          F      --    Flexural modulus                    1300       3100   --    (kgf/cm.sup.2)    W#1 Carry (yard)                    217        216    220    Shot feel       x          x      Δ    Controllability at                    Δ    X      Δ    approach shot    ______________________________________     *1: twopiece golf ball, manufactured by Sumitomo Rubber Industries., Ltd.

As is apparent from the comparison of the physical properties of thegolf balls of Examples with those of the golf balls of ComparativeExamples, the golf balls of Examples show a carry flight distancefarther than the golf balls of Comparative Examples, and have bettershot feel and better controllability at approach shot than the golfballs of Comparative Examples.

Test Method

(a) Carry

The golf balls composed of the core, the inner cover layer and the outercover layer comprising the compositions described above were produced.After a No. 1 wood club W#1 was mounted to a swing robot manufactured byTrue Temper Co. and the golf ball was hit at a head speed of 45m/second, a carry was measured as a flight distance.

(b) Shot feel and controllability

The shot feel and controllability at approach shot of the golf ball wereevaluated by 10 professional or high level amateur golfers according toa practical hitting test. The evaluation criteria are as follows.

(Evaluation criteria):

∘: Not less than 8 out of 10 golfers felt that the golf ball has goodshot feel or good controllability.

Δ: From 4 to 7 out of 10 golfers felt that the golf ball has good shotfeel or good controllability.

x : Not more than 3 out of 10 golfers felt that the golf ball has goodshot feel or good controllability.

EFFECT OF THE INVENTION

In the solid golf ball of the present invention, rebound characteristicscan be restrained from degrading when reducing the core hardness, byusing the core obtained by molding the compositions described above, anddurability, shot feel at the time of hitting and controllability atapproach shot can be improved by using multi-layer structured cover.

What is claimed is:
 1. A solid golf ball comprising a core and a coverformed on the core, wherein the cover has a two-layer structureconsisting of an inner cover and an outer cover formed on the innercover, and the core is formed from a rubber composition comprising 0.05to 5 parts by weight of an organic sulfide compound, based on 100 partsby weight of a base rubber, and the core has a deformation amount of 3.5to 6.0 mm, when applying from an initial load of 10 kg to a final loadof 130 kg on the core.
 2. The solid golf ball according to claim 1,wherein the inner cover has a flexural modulus of 3,000 to 7,000kgf/cm².
 3. The solid golf ball according to claim 1, wherein the outercover has a flexural modulus of 1,000 to 2,800 kgf/cm².